Tubular or rod-shaped machine elements are used as shafts or hollow shafts, axes or rod-structures in many fields of machine and plant construction. A comprehensive and precise knowledge about the mechanical stresses acting on the machine elements during fabrication and assembly is of great interest both during the development phase of a new machine element and during its use and the entire service life of the machine element.
In order to measure the mechanical forces and moments acting on the machine element particularly in test stands and development experiments suitable sensors are fixed from the outside on a surface of the machine element and the sensor values are measured and evaluated. In many cases, strain gauges can be used to detect even minor deformations on the surface of the machine element in a cost-effective and reliable manner.
A continuous monitoring of sensor values is beneficial and sometimes required for example, in the weighing technology, load monitoring as well as in production control and production monitoring. In addition to permanent condition monitoring, sensor devices and application possibilities are also known by which structural damages such as cracks or plastic deformation can be detected at an early stage, which can lead to an impairment of the function of the machine element or damage and reduce the life and reliability of the machine element. Depending on the changes or the structural damage detected by the sensor device, it may be possible to detect an imminent failure of the machine element before a critical condition occurs.
In order to enable the most reliable and precise measurement of the mechanical stresses affecting the machine element, it is necessary to fix the sensor device on the machine element as precisely and permanently as possible. At the same time, it should be avoided that the sensor equipment is subjected to excessive stress during the measurement period, during its intended use and also during manufacture and assembly, which could lead to incorrect measurements or damage to the sensor device. At the same time, compared to a conventional machine element, the sensors should not cause any restrictions on handling and use, such as those caused by sensors mounted on an outside of the machine element or by joints that may be required for mounting sensors.
For example, it is known from EP 1 597 128 B1 that the sensor device is arranged in a cavity of a shaft to protect it from external influences and environmental conditions. The sensor device is protected in the cavity of the shaft against operational mechanical and schematic influences as well as against any willful damage. Vibration and structure-borne noise of the machine element can be recorded and evaluated with the sensor device arranged in the cavity. A permanently precise fixture of the sensor device in the cavity is not necessary, since the sensor device is not intended to measure the deformation of the cavity of the machine element.
Hollow shafts are known where a sensor device is fixed in an interior of the hollow shaft by adhesive bonding or press-fit force-locking. Although the position of the sensor device within the hollow shaft can be fixed relatively accurately, sensor devices arranged in this way are only suitable to a limited extent for the comprehensive detection of axial and radial deformations of the hollow shaft, since a reliable analysis of sensor signals usually requires that the relevant sensor or the sensor device is fixed with a preload at the measuring point. Furthermore, a significant amount of installation work is required for the arrangement and definition of the sensor equipment.